The existing data say "partly" rather than "largely." There is still a substantial influence of environmental factors found in any study of "broad heritability" (which is a characteristic of groups, not of individuals) of IQ. And at the individual level, there is a very large malleability of IQ through the best known interventions. Important data showing the powerful effect of environmental influences on IQ include the trends in raw scores on IQ tests over time in most countries of the world, resulting in the need to renorm IQ tests every half generation or so.

All these data and more can be found cited to primary sources in Intelligence and How to Get It: Why Schools and Cultures Count by Richard E. Nisbett,

http://www.amazon.com/Intelligence-How-Get-Schools-Cultures/...

a recently published book that is well worth a read.

Edit: full URL is http://books.google.com/books?id=YdnK_PLtDVQC&pg=PA33...

1. Smart parents -> Give their kids more exposure -> More Exposure and guidance make the kids smart

2. Smart parents -> kids inherit their smartness.

Greg was talking about genes , or case 2, and not case 1. which is what I think is unsubstantiated.